This invention relates to an electric drive system for a vehicle.
Vehicle electric drive systems or AC electric traction drives have been proposed to overcome some of the deficiencies of mechanical transmission systems, such as a limited number of speeds, increased costs of engineering and manufacturing components, and limiting vehicle configuration options. Such an electric drive system, as shown in U.S. Pat. No. 5,568,023 issued Oct. 22, 1996 to Grayer et al., typically includes an engine-driven 3-phase electric motor/generator coupled to an inverter/rectifier, which, in turn, is coupled to a DC bus. The bus feeds an inverter/rectifier which supplies power to a traction motor/generator which drives an axle or a wheel. The inverter/rectifiers invert the DC current on the bus to 3-phase AC current at a frequency to drive the wheels at the speed directed by the operator. An external power source applied to the tractor through the drive wheels and tending to move the tractor at a speed faster than the requested speed will cause the motors to act as generators and the whole sequence of power conversion will be reversed, regenerating mechanical power back into the engine. This regeneration action causes the engine to absorb power from externally forced loads in a manner similar to that of current mechanical transmissions.
Typically, the speed of the traction motor/generators is controlled by controlling the frequency of the current driving the motor. When the speed control is engaged, the drive will engage with full force or torque authority. Operators of conventional tractors with mechanical transmissions can depress a clutch pedal to release or reduce the torque driving the vehicle. By slowly engaging or disengaging such a mechanical clutch, the operator can control the torque being applied by the engine to move the vehicle. Therefore, by modulating the engagement of the clutch, the operator controls movement of the vehicle by controlling the driving force or torque that the wheels can exert. It would be desirable to have a similar clutch type control capability in an electric drive system.
Accordingly, an object of this invention is to provide a vehicle electric drive system with a control which operates in a manner similar to a clutch control of a conventional mechanical drive vehicle.
These and other objects are achieved by the present invention, wherein a vehicle electric drive system includes an engine driven electric motor/generator, a first inverter/rectifier coupled to motor/generator, a bus coupled to the first inverter/rectifier, a second inverter/rectifier coupled to the bus, and a traction motor/generator coupled to an output of the second inverter/rectifier. Electronic controllers control operation of the inverter/rectifiers in response to an operator speed control member. In addition, an operator controlled foot pedal is coupled to a transducer which generates a limit command signal representing the position of the foot pedal. An electronic control unit receives the limit command signal and limits current supplied by the second inverter/rectifier to the traction motor/generator to a limit current which is a function of the limit command signal and motor speed.